1. Field of the Invention
The invention is related to the field of metrology, and in particular, to an x-ray analysis system capable of efficient measurements on semiconductor wafers.
2. Related Art
As semiconductor devices continue to shrink to enable greater device density in integrated circuits (ICs), accurately and efficiently measuring the characteristics of the structures that form those devices becomes increasingly difficult. In the realm of thin film composition and thickness measurements, x-ray analysis metrology systems (i.e., metrology systems that measure x-ray emissions from a thin film) are becoming increasingly important for ensuring that semiconductor wafers have been properly processed.
For example, modern gate dielectrics are typically silicon dioxide (SiO2). A small amount of nitrogen is sometimes added to improve the electrical characteristics of the gate dielectric. By directly measuring nitrogen x-ray emissions from a gate dielectric layer, an x-ray analysis metrology system can determine if the oxidation process is providing the proper nitrogen concentration in a thin (e.g., 20 A or less) gate dielectric.
The two main x-ray analysis metrology techniques are electron probe microanalysis (EPMA) and x-ray fluorescence (XRF). EPMA is a metrology technique in which an electron beam (e-beam) is directed at a thin film to cause the thin film to emit x-rays. Those emitted x-rays can then be analyzed to determine the composition and/or thickness of the thin film. In XRF, an x-ray beam is used instead of an e-beam to generate x-ray emissions from the thin film. Both techniques can provide the type of high-precision compositional analysis capabilities required to evaluate modern semiconductor device structures.
FIG. 1A shows a conventional EPMA system 100 that includes an e-beam source 110, a stage 140 for supporting a test sample 120, a cylindrical crystal (or multilayer) diffractor 130, and an x-ray detector 150. E-beam source 110 directs an e-beam 111 at an analysis spot 125 on test sample 120, thereby causing test sample 120 to emit output x-rays 121.
Note that for clarity, only a portion of output x-rays 121 emitted from test sample 120 are depicted. The actual x-ray emission from test sample 120 in response to e-beam 111 will occur in all directions from analysis spot 125. A portion of those output x-rays 121 are reflected and focused onto x-ray detector 150 by diffractor 130 so that the characteristics (e.g., elemental origin and quantity/ratio) of those output x-rays 121 can be measured. The measurements taken by x-ray detector 150 can then be used to determine the composition and/or thickness of a thin film on test sample 120.
Diffractor 130 and x-ray detector 150 form what is sometimes referred to as a wavelength-dispersive x-ray (WDX) detector. Diffractor 130 is tuned to only reflect a particular x-ray wavelength, which allows x-ray detector to precisely measure the level of a particular element (i.e., the element that generates the x-ray wavelength for which diffractor 130 is tuned) within test sample 120.
High performance XRF systems also sometimes incorporate WDX detectors to provide high-precision measurement capabilities. An XRF system incorporating a WDX detector would operate in substantially the same manner as described above with respect to EPMA system 100, except that e-beam source 110 would be replaced with an x-ray generator for directing a focused x-ray beam (rather than e-beam 111) at analysis spot 125. Collection and measurement of the resulting output x-rays 121 would be performed by diffractor 130 and x-ray detector 150 in the same manner as described above with respect to EPMA system 100.
The speed at which measurements can be taken by EPMA system 100 (or a comparable XRF system) is dependent on the x-ray flux at x-ray detector. Therefore, the larger the amount of x-ray emission that can be reflected and focused by diffractor 130 (onto x-ray detector 150), the more quickly EPMA system 100 can complete a measurement on test sample 120.
Unfortunately, diffractor 130 is not well suited for intercepting a large portion of the total x-ray emission from test sample 120. Diffractor 130 is formed from multiple layers of parallel crystal planes. Incoming x-rays that exhibit incident angles that are very near the Bragg angle are partially diffracted by the multiple crystal planes. X-rays having wavelengths that are integer multiples of the distance between the crystal planes experience constructive interference at diffractor 130, and therefore provide a strong response at x-ray detector 150.
Diffractor 130 can only reflect x-rays that exhibit incident angles with the diffractor that are very near the Bragg angle (the Bragg angle is determined by the x-ray energy and the spacing between crystal planes in the diffractor). Therefore, diffractor 130 can only span a very small arc of the Rowland circle before it can no longer reflect the desired x-ray wavelengths. As a result, the x-ray flux at x-ray detector 150 is relatively low, and metrology operations using EPMA system 100 (and similar XRF systems) can be very time consuming. This throughput problem is exacerbated for thin films that generate relatively low concentrations of the x-ray wavelength of interest (e.g., the low-concentration nitrogen x-rays emitted from a thin gate dielectric layer).
The time consuming nature of conventional x-ray metrology tools has mandated that such tools be used as “off-line” tools in production environments. For example, FIG. 1B shows an exemplary flow diagram for a conventional EPMA tool in a production environment. In a “PERFORM FIRST PROCESS” step 181, a batch (e.g., a cassette) of wafers is processed (e.g., gate oxides are formed on the wafers). A monitor wafer is then selected from the processed batch in a “SELECT MONITOR WAFER” step 182 to begin the metrology operation. An e-beam (111) is then directed at the wafer in a “DIRECT E-BEAM AT MONITOR WAFER” step 183, and the resulting x-rays (121) are focused by a diffractor (130) at an x-ray detector (150) in a “FOCUS X-RAYS W/DIFFRACTOR” step 184. The focused x-rays (131) are then measured by the x-ray detector (150) in a “MEASURE FOCUSED X-RAYS” step 185, and the desired characteristics of the test sample are then determined in a “DETERMINE MONITOR WAFER PROPERTIES” step 186. If additional monitor wafers from the batch of processed wafers are to be evaluated, the process then loops back to step 182. The results of the EPMA measurement(s) on the monitor wafer(s) can then be used to determine if the first process is performing within specification in a “QUALIFY FIRST PROCESS” step 187.
Note that because the EPMA operation(s) of steps 182 through 187 is relatively time consuming (for the reasons described above with respect to FIG. 1A), processing of the batch of wafers from which the monitor wafer(s) being examined in steps 182 through 187 has been selected continues in parallel with that analysis in a “PERFORM SECOND PROCESS” step 190. Therefore, the EPMA analysis performed in steps 182 through 187 is described as an “offline analysis”. Unfortunately, offline analysis is generally an undesirable technique, because by the time a problem is discovered by the offline analysis, significant additional (costly) processing may have been performed on the problematic batch of wafers. Furthermore, the additional processing can make subsequent tracing of the root cause of the problem impossible.
Accordingly, it is desirable to provide a system and method for efficiently performing x-ray analysis metrology.